To make Topping: Pulse pistachios in food processor until ground, but with some larger pieces. Transfer to bowl, and stir in olive oil, garlic, and rosemary.
To make Focaccia: Stir together flour, sugar, yeast, salt, and 1 1/4 cups water in bowl. Knead dough in bowl 5 minutes. Cover bowl with towel, and let rise 1 hour, or until doubled in size.
Generously grease rimmed baking sheet with olive oil. Stretch dough into 12- x 11-inch rectangle in sheet pan. Let dough rise 1 hour.
Preheat oven to 450°F. Dimple dough with fingers. Whisk 2 Tbs. water into pistachio mixture, and brush onto dough. Press tomato halves cut-side up into dough. Bake 25 to 30 minutes, or until golden brown on top and bottom.